Oil production by in situ combustion



States This invention relates to a process for producing hydrocarbons from a carbonaceous stratum by in situ conbustion.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial proportion of the hydrocarbon in the stratum drives out and usually upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein followed by injection of air thru the ignition borehole and recovery of product hydrocarbons and combustion gas thru another borehole in the stratum is a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difficulty and to permit the continued progress of the combustion zone thru the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the combustion zone from one or more surrounding boreholes.

In situ combustion techniques are being applied to tar sands, shale, Athabasca sand and other strata in virgin state, to coal veins by fracturing and to strata partially depleted by primary and even secondary and tertiary recovery methods.

One of the problems in producing hydrocarbons by in situ combustion lies in establishing a combustion zone in the stratum which can be propagated therethru by feeding a combustion supporting gas either directly or inversely to the combustion front. The stratum in the area to be ignited must be raised in temperature to at least the ignition temperature of the hydrocarbon material in the stratum and, because of the heat loss to surrounding cold stratum, it is difiicult to establish a sufiicient heat reservoir in the area to be ignited to permit ignition of the carbonaceous material and propagation of the resulting combustion zone thru the stratum.

It is also found that too high a temperature in the combustion zone results in excessive destructive cracking of the hydrocarbon material, particularly in the inverse injection technique wherein all of the produced hydrocarbons are forced thru the hot combustion zone and the hot sand or rock behind the combustion zone.

Another problem encountered in the production of hydrocarbon by in situ combustion is the slow rate of propagation of the combustion front thru the stratum by known methods in this field. Of course, the rate of propagation of the combustion front determines the rate of producing hydrocarbons and has a material effect on the efiiciency and economy of the process.

Accordingly, it is an object of the invention to provide an improved process for producing hydrocarbons from a carbonaceous stratum by in situ combustion. Another object is to lower the ignition temperature of the carbonatet aceous material when igniting a carbonaceous stratum preparatory to producing the same by in situ combustion. Another object is to lower the combustion temperature during the in situ combustion process so as to minimize the destructive cracking of the hydrocarbon material, particularly, by movement of the combustion front by inverse air injection. A further object of the invention is to increase the rate of propagation of the combustion front thru a carbonaceous stratum. Other objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises injecting a compound of the group consisting of formaldehyde and paraformaldehyde into a stratum so as to deposit paraformaldehyde in a section of stratum to be ignited and produced, heating and igniting the impregnated stratum with a combustion supporting gas, and burning a portion of the carbonaceous material in the stratum so as to produce hydrocarbons therefrom. The paraformaldehyde deposited in the stratum burns in the ignited area and has the effect of reducing the ignition temperature and of increasing the propagation rate of the resulting combustion front thru the stratum.

The paraformaldehyde and/ or formaldehyde is injected into the stratum to be produced preferably by passing a carrier gas over paraformaldehyde in particulate form maintained at a temperature sufficient to vaporize and/or sublime the paraformaldehyde at a suitable rate to effect a concentration of the compound in the carrier gas in the range of about 0.01 to 1.5 volume percent. The desired vaporization or sublimation may be eflfected either by regulating the temperature of the carrier gas or by maintaining the compound at the desired temperature. Paraformaldehyde sublimes at about 46 C. and boils at about 64 C. when maintained under pressure. Air is the preferred carrier gas but other gases inert with respect to the formaldehyde or paraformaldehyde, such as steam, CO nitrogen, combustion gas, normally gaseous hydrocarbons, etc., may be utilized either alone or in admixture. The paraformaldehyde may be placed in the well in the stratum in which the formaldehyde is to be deposited and there contacted with the hot carrier gas before the gas is passed into the stratum. In this manner the carrier gas carries formaldehyde into the adjacent stratum and deposits the formaldehyde therein.

It is also feasible to vaporize formaldehyde from a solution thereof by passing the warm carrier gas therethru at a rate which produces the desired concentration of the aldehyde in the gas. The formaldehyde polymerizes to paraformaldehyde on the reservoir rock surfaces.

When utilizing the invention in connection with a ringtype well pattern around a central well wherein the ignition is initiated around the central well and the resulting combustion front is driven to the ring of wells, the carrier gas may be injected either thru the wells in the ring and vented thru the central well or thru the central well and vented thru the wells in the ring. In either case there are two methods of determining the time for terminating the injection phase of the process. One method comprises continuing the injection until the permeability of the stratum drops appreciably, such as at least about 10 percent. The other method comprises injecting the carrier gas until the concentration of the formaldehyde or paraformaldehyde in the vent gas amounts to at least 15 percent of the original concentration. Generally higher concentrations of the formaldehyde in the stratum are more effective in producing the benefits of the invention. The cost of the formaldehyde places a limit on the amount of deposition which is economical.

A preferred method of effecting the deposition of formaldehyde comprises injecting the carrier gas-formaldehyde mixture thru the central ignition well in a ring type pattern and venting the carrier gas thru the wells in the ring until the concentration of the formaldehyde in the vent gas reaches the required minimum or the permeability between the central well and the wells in the ring drops about percent or more. The stratum around the central well is then heated to ignition temperature by any suitable method, such as by burning a charcoal pack in the well adjacent the stratum and passing combustion supporting gas into the heated stratum around the ignition well so as to ignite the same. The combustion supporting gas may be injected either thru the ignition well or thru the wells in the ring, but it is preferred to use the latter technique and incorporate in the injected air a concentration of fuel gas, such as normally gaseous hydrocarbons in the range of 1 to 2 or 3 percent by volume of the air. When injecting air thru the Wells in the ring, the combustion front established around the central well is caused to move countercurrently to the flow of air thru the stratum to the injection wells. As the fire front advances, the formaldehyde deposited on the sand or rock in the stratum burns along with hydrocarbon material and the injected gas carries some of the formaldehyde into the combustion zone. In this manner the deposited compound has the effect of reducing combustion temperature and increasing the velocity or rate of propagation of the combustion zone thru the stratum.

The invention may also be used in effecting recovery of hydrocarbons by in situ combustion in an in-line well pattern wherein combustion is initiated around a plurality of wells in a line and the resulting combustion front is driven by either inverse air injection or by direct air injection to a second line of wells generally parallel with the line of ignition wells.

To illustrate the invention, the deposition of formaldehyde in a carbonaceous stratum was effected by passing 'air over Va x A2. inch paraformaldehyde pellets maintained at about 41 C. The carrier gas contained formaldehyde in a concentration of 0.156 volume percent. This gas mixture was passed thru a tar sand pack in a tube maintained at about 75 F. using a gas velocity of 200 standard cubic feet per hour per square foot of cross section. The exhausted gas had a formaldehyde concentration of only 0.066 volume percent. Of course the amount removed from the carrier gas remained on the tar sand. The concentration of formaldehyde in the vent gas rose slowly as injection of the mixture continued, indicating a trend to ward saturation of the sand surface with the formaldehyde.

In igniting and burning a tar sand from the Bellamy, Missouri area by the counterfiow technique with formaldehyde in the combustion zone it was found that the formaldehyde effected a decrease of about 7.5 percent in the combustion temperature and an increase of about 10.5 percent in the propagation velocity. The average temperature in the combustion area was 915 F. with formaldehyde present as compared with a temperature of 990 F. in the absence of formaldehyde. The average propagation velocity in feet per hour was 0.648 with formaldehyde present as compared with 0.586 in the absence of formaldehyde.

There is a unique advantage in combining the deposition of formaldehyde in the stratum being produced by in situ combustion and production by inverse air injection because of the fact that when utilizing the inverse technique all of the hydrocarbon material is passed thru the hot combustion zone and the hot rock behind this zone. By reducing the temperature of the combustion with formaldehyde, the produced hydrocarbons are not subjected to the usually high combustion temperatures of the conventional inverse injection process thereby substantially reducing the amount of destructive cracking of the hydrocarbons in proportion to the lowering of the temperature of combustion. This benefit is in addition to the increased propagation velocity and the higher efficiency effected by the lower temperature in the amount of heat required to effect the process, which heat comes from the burning of hydrocarbon material in the stratum. Naturally the less hydrocarbon material burned in producing the stratum, the more the hydrocarbon recovered.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative deails disclosed are no to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for producing hydrocrabons from a subterranean carbonaceous stratum penetrated by a pair of spaced apart wells, comprising contacting paraformaldehyde with a stream of hot gas so as to entrain formaldehyde in said gas; injecting the resulting gas thru one of said wells into a section of stratum adjacent said well so as to deposit formaldehyde in the pores of said section until the permeability thereof is lowered substantially; thereafter heating the impregnated section of stratum around one of said wells to combustion-supporting temperature and injecting combustion-supporting gas into said section thru the other of said wells so as to burn said formaldehyde and ignite said stratum at a lower temperature than the ignition temperature without said formaldehyde; continuing injection of said combustion-supporting gas thru said other well so as to move the resulting combustion zone toward said other well; and recovering produced hydrocarbons thru said one of said wells.

2. The process of claim 1 comprising positioning a mass of paraformaldehyde downhole adjacent the stratum to be ignited and in the well thru which said hot gas is injected so that resulting entrained formaldehyde goes directly into the adjacent section of stratum.

3. The process of claim 2 wherein said gas comprises air and its temperature is maintained below the ignition temperature of said stratum prior to deposition of said formaldehyde.

4. A process for producing hydrocarbons from a carbonaceous stratum comprising passing warm carrier gas over paraformaldehyde so as to vaporize same and produce a concentration of formaldehyde in said gas in the range of 0.01 to 1.5 volume percent; thereafter injecting said gas into said stratum so as to deposit said formaldehyde in the pores of said stratum around an ignition well therein until the permeability thereof is lowered substantially; igniting said stratum around said ignition well; injecting combustion-supporting gas thru at least one ofiset injection well to feed the combustion area and move a combustion front thru said stratum toward said injection well, whereby said formaldehyde is burned and the velocity of propagation of said front is increased and the normal combustion temperature of said stratum is lowered; and recovering produced hydrocarbons thru said ignition well.

5. The process of claim 4 wherein said gas is injected thru said ignition well so as to deposit said formaldehyde in an annulus of said stratum extending at least several feet from said ignition well and carrier gas is vented thru said injection well.

6. The process of claim 5 wherein injection of carrier gas containing said formaldehyde is continued until the concentration of said compound in the vent gas is at least 5 percent of the injected concentration.

7. The process of claim 4 wherein injection of carrier gas containing said formaldehyde is continued until the permeability of said stratum between the ignition well and the injection well has dropped at least 10 percent.

5 6 said stratum between the injection and ignition wells has 2,793,696 Morse May 28, 1957 dropped at least 15 percent. 2,871,942 Garrison Feb. 3, 1959 2,931,437 Smith Apr. 5, 1960 References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS 5 Walker, I. F; Formaldehyde, 2nd edition, Reinhold 2,722,277 Crawford Nov. 1, 1955 Pub, C0rp., 1953,pages 1 and 41-43. 

1. A PROCESS FOR PRODUCING HYDROCARBONS FROM A SUBTERRANEAN CARBONACEOUS STRATUM PENTRATED BY A PAIR OF SPACED APART WELLS, COMPRISING CONTACTING PARAFORMALDEHYDE WITH A STREAM OF HOT GAS SO AS TO ENTRAIN FORMALDEHYDE IN SAID GAS; INJECTING THE RESULTING GAS THRU ONE OF SAID WELLS INTO A SECTION OF STRATUM ADJACENT SAID WELL SO AS TO DEPOSIT FORMALDEHYDE IN THE PORES OF SAID SECTION UNTIL THE PERMEABILITY THEREOF IS LOWERED SUBSTANTIALLY; THEREAFTER HEATING THE IMPREGNATED SECTION OF STRATUM AROUND ONE OF SAID WELLS TO COMBUSTION-SUPPORTING TEMPERATURE AND INJECTING COMBUSTION-SUPPORTING GAS INTO SAID SECTION THRU THE OTHER OF SAID WELLS SO AS TO BURN SAID FORMALDEHYDE AND IGNITE SAID STRATUM AT A LOWER TEMPERATURE THAN THE IGNITION TEMPERATURE WITHOUT SAID FORMALDEHYDE; CONTINUING INJECTION OF SAID COMBUSTION-SUPPORTING GAS THRU SAID OTHER WELL SO AS TO MOVE THE RESULTING COMBUSTION ZONE TOWARD SAID OTHER WELL; AND RECOVERING PRODUCED HYDROCARBONS THRU SAID ONE OF SAID WELLS. 